Usage has changed in 0.3.0.

1. include the 0.11 render pass, DrawImguiDesc, with the appropriate config.
2. Render windows using the amethyst_imgui::with function.

amethyst_imgui::with(|ui| {
	ui.show_demo_window(&mut true);
});

No systems or other shinanigans are required now. All imgui state management occurs within the imgui render pass.